TRANSFORMATION OF SYNTHETIC MACKINAWITE TO HEXAGONAL PYRRHOTITE - A KINETIC-STUDY

The kinetics of the transformation of synthetically prepared mackinawi te to hexagonal pyrrhotite have been studied to clarify the relationsh ip between these two phases. Kinetic analysis using the Johnson-Mehl e quation is based on measurements of the fraction of hexagonal pyrrhoti te obtained by heating mackinawite samples in the temperature range of 530-545 K. This gives isothermal rate constants from which an activat ion energy of 493 kJ/mol and a frequency factor of 3.7 x 10(45) min(-1 ) have been established. The mechanism proposed for this transformatio n is that of a solid-state diffusion process. Kinetic data derived fro m this experiment may be used to show that persistence of mackinawite in nature can be explained by kinetic factors alone. Reactions at low temperatures, in which mackinawite is oxidized or further sulfidized, are therefore more important than the kinetically inhibited transforma tion to hexagonal pyrrhotite. The irreversible mackinawite to hexagona l pyrrhotite relation is compared with the reversible tetragonal FeSe to NiAs-type FeSe phase transformation. This contrast in stability beh avior is examined in terms of the thermal stability of the respective tetragonal phases relative to the upper magnetic ordering temperatures of their corresponding hexagonal phases.